The field of the invention is particle beam systems, in particular, electron beam systems.
When running electron beam or other particle beam systems, it is necessary to correct for drift, electrical and/or thermal, in the operating parameters of the system. Accordingly, it is conventional to direct the beam at a target in a known location and to correct the system parameters as required in order to restore correct operation of the system.
Conventionally, in the particular field of integrated circuit fabrication, the target is composed of a grid of gold on an integrated circuit wafer of silicon. Silicon is conventionally used because it is the material of the workpiece that is being operated on, so that the parameters of the target are similar to the operating parameters in system operation.
In particular, the surface of a silicon substrate will have the same degree of smoothness as the wafer being patterned.
The degree of smoothness is important because a high quality target image is needed to obtain an accurate calibration. If the surface is not smooth, the fidelity of the target image may be compromised, with large and random variations in the width, and even discontinuities if the surface topography is sufficiently rough. The depth of focus of an electron beam image is very small, being about a few microns for current technology. An out of focus beam will produce an exposed area of resist that is too large and may produce a discontinuity if the image is so diffuse that the development threshold of the resist is not met. Discontinuities may also be produced by step coverage problems.
U.S. Pat. No. 4,163,900 shows the use, in an X-ray analysis system, of a grid substrate that is cast from a composite material of low atomic number, including carbon fibers. The line widths used were about 150 xcexcm, qualitatively different from today""s line widths of about 150 nm by three orders of magnitude.
The invention relates to a calibration target for particle beam systems that employs carbon as the substrate, gold as the scattering material and an intermediate smoothing layer deposited on the substrate that brings the irregularities of the substrate within tolerance.
An alternative version of the invention employs a gold background carbon grid.